The advent of the age of electronic information has brought with it a need for printers able to print faster and with a higher print density. Laser printers, which employ a laser light source, and light emitting diode (LED) printers, which employ an LED array as the light source, are two examples of printers used in response to such needs. While a laser printer requires the use of a mechanical mechanism, such as a rotating polygonal mirror, for the scanning laser beam with an LED printer, it is only necessary for the light-emitting diodes (hereinafter also referred to as "light-emitting elements") that make up the light-emitting diode array to be electrically controlled. The advantages of LED printers compared with laser printers are that they do not have any mechanical moving parts, and therefore can be made smaller than laser printers and are faster and more reliable.
Referring now to FIG. 1 of the drawings, there is shown a cross-section view of a very typical example of a prior art conventional LED array 50. For simplicity only two light-emitting elements 58 are shown, but the array 50 can include any number of the light-emitting elements 58. The array 50 comprises a substrate 51 of n-type conductivity GaAs having on a surface 51a thereof a layer 52 of n-type conductivity GaAsP. The GaAsP layer 52 is typically about 15 microns thick and can be deposited on the substrate 51 by vapor-phase epitaxy (VPE). A masking layer 53 of SiN.sub.x is on the GaAsP layer 52 has a pair of spaced openings therethrough. P-type conductivity diffused regions 54 are in the GaAsP layer 52 at each of the openings 59 in the masking layer 53. The diffused regions 54 are formed by diffusing zinc into the GaAsP layer 52 through the openings 59 to a depth of about 1.5 microns. A p-n junction between each of the diffused regions 54 and the GaAsP layer 52 forms a separate light-emitting element 58. A separate electrode 55 is on each of the diffused regions 54 and an electrode 56 is on a surface 51b of the substrate 51. An antireflection layer 57 of SiN.sub.x is over the masking layer 53, the exposed areas of the diffused regions 54 and the electrodes 55. The antireflection layer 57 has an opening therethrough (not shown) over a portion of the array 50 which does not contain a light-emitting element 58 to form a bonding pad for the electrodes 55.
A problem encountered when the light-emitting diode array 50 is used as a printer light source, unlike when individual LEDs are used, is that of variation in characteristics from element 58 to element 58. In the conventional example shown in FIG. 1, the light-emitting region formed by the diffused region 54 is formed simply by using zinc diffusion, thereby reducing variation caused by non-uniformities in the fabrication process.
However, light-emitting elements 58 thus formed contain high-density lattice defects which arise from a lack of lattice matching between the n-type conductivity GaAsP layer 52 and the GaAs substrate 51. This produces considerable non-uniformity of the material itself and a low emission efficiency. Moreover, the light-emitting region is a p-n homojunction which is not the most suitable type from the standpoint of emission efficiency.
Referring now to FIG. 2, there is shown a cross-sectional view of a prior art AlGaAs single heterojunction type light-emitting diode array 60 which overcomes the drawbacks of the conventional GaAsP light-emitting diode array 50, shown in FIG. 1. The array 60 comprises a p-type conductivity GaAs substrate 61 having on a surface 61a thereof a layer 62 of p-type conductivity Al.sub.x Ga.sub.1-x As. The layer 62 is about 10 microns in thickness and contains a doping concentration of zinc of about 5.times.10.sup.17 impurities/cm.sup.3. On the p-type layer 62 is a layer 63 of n-type conductivity Al.sub.y Ga.sub.1-y As. The layer 63 is of a thickness of about 5 microns and contains a doping concentration of Te of about 8.times.10.sup.17 impurities/cm.sup.3. Spaced grooves extend through the n-type conductivity layer 63 and a portion of the p-type conductivity layer 62 to form separated mesa-shaped light-emitting regions 69. On a portion of the n-type conductivity layer 63 of each of the light emitting regions 69 is a layer 64 of n+ type conductivity GaAs. The layers 64 are of a thickness of about 0.1 microns and are doped with Sn to a concentration of about 5.times.10.sup.18 impurities/cm.sup.3. For emitting light with a wavelength in the region of 720 nm, the aluminum composition in the layers 62 and 63 is x=0.2 and y=0.5.
A protective layer 66 of SiN.sub.x is over the surfaces of the grooves. A separate electrode 65 is on each of the GaAs layers 64 and an electrode 67 is on a surface 61b of the substrate 61. An antireflection layer 68 of SiN.sub.x is over the protective layer 66, the surface of the layers 63 and the electrodes 65.
The array 60 is made by epitaxially depositing on the substrate 61 in succession the p-type layer 62, the n-type layer 63 and the GaAs layer 64. This can be achieved by liquid-phase epitaxy. A masking layer, not shown, of SiN.sub.x is then deposited over the GaAs layer 64 and defined to extend over the areas which are to be the mesas using standard photolithography and plasma etching. Using a chemical etching with H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O=1:8:80 the exposed areas of the GaAs layer 64 and the underlying areas of the p-type conductivity layer 63 and n-type conductivity layer 62 are removed to form the grooves 68. The grooves 68 may extend 1 micron or more into the n-type layer 62.
The protective layer 66 is then deposited by plasma CVD and is etched away from over the top surfaces of the light-emitting regions 69. The electrodes 65 and the electrode 67 are formed by vapor deposition. The unnecessary portions of the electrodes 65 are then removed by photolithography and plasma etching. Using a chemical etchant of NH.sub.4 OH:H.sub.2 O.sub.2 =1:10, the GaAs layer 64 is removed except for the portion under the n-side electrodes 65. Finally, the antireflective layer 68 is formed by plasma CVD, and the device is heated to alloy the electrodes 65 and 67.
Structurally, the array 60 is an array of conventional high-luminance LEDs. This arrangement of the array 60 suppresses optical energy attenuation caused by internal absorption by using the n-type conductivity Al.sub.y Ga.sub.1-y As layer 63 which is transparent to light emitted by the p-type conductivity Al.sub.x Ga.sub.1-x As layer 62. Also, as well as using a junction with good crystalline growth qualities, the heterojunction improves the carrier injection efficiency and results in an overall external output efficiency that is several times higher than that achievable with the light-emitting diode array shown in FIG. 1.
While the type of light-emitting diode array 60 shown in FIG. 2 has excellent characteristics, the necessity of suppressing optical crosstalk between elements means the n-type conductivity Al.sub.y Ga.sub.1-y As layer 63 used as a window has to be completely removed between elements, and non-mesa portions of the p-type Al.sub.x Ga.sub.1-x As layer 62 that form the emission region have to be etched down to a certain minimum depth to reduce optical bleeding.
The diffusion length of minority carrier electrons injected into the p-type Al.sub.x Ga.sub.1-x As layer 62 decreases with the distance from the p-n junction. Since this distance is in the order of 10 microns, at least about 10 microns of the p-type conductivity Al.sub.x Ga.sub.1-x As layer 62 has to be etched away. However, it is difficult to etch that deep with adequate process uniformity and reproducibility. Thus, some degree of optical bleeding is unavoidable.
Furthermore, the p-type Al.sub.x Ga.sub.1-x As layer 62 that is within the diffusion length of electrons from the p-n junction functions effectively as an emission layer. Thus, in order to optimize the emission efficiency, it is necessary to make the p-type Al.sub.x Ga.sub.1-x As layer 62 at least 10 microns thick. A problem is, however, that even if the mission efficiency is improved, owing to the high refractive index of the light-emitting portion, most of the light is lost through total reflection. This results in a very low external light output efficiency of no more than several percent. Yet another problem in the case of a light-emitting diode array is that the priority is on reliability and reproducibility, while the small size of the discrete elements makes it exceedingly difficult to directly mount a lens or the like on individual elements.
Referring now to FIG. 3, there is shown a cross-sectional view of a light-emitting diode array 82 which we have previously proposed in which an optical output surface is processed to prevent reflection. The array 82 comprises a substrate 70 of n-type GaAs having on a surface 70a thereof a layer 71 of n-type GaAsP. The layer 71 is about 15 microns in thickness and is epitaxially deposited on the substrate 70 by vapor phase epitaxy. On a surface of the layer 71 is a masking layer 74 of SiN.sub.x having a pair of a spaced apart openings 83 therethrough. P-type conductivity diffused regions 76 are in the layer 71 at each of the openings 83 in the masking layer 74. Diffused regions 76 are formed by diffusing zinc through the openings 83 into the layer 71. Projections 78 are formed along the surface of the diffused regions 76. The pitch of the projections 78, i.e., the spacing between the projections 78, ranges from less than a micron to several microns. The projections 78 are at an inclination of 45.degree. and are only formed in a direction perpendicular to the [011] orientation of the GaAsP. The projections 78 are formed using photolithography and etching. A separate electrode 80 is on each of the diffused regions 76 and an electrode 81 is on a surface 70b of the substrate 70.
In the array 82, the inclination of the projections 78 changes the angle at which light generated in the diffused regions 76 is incident on the surface of the diffused regions 76. This makes it possible to achieve an external light output even at an angel at which, previously, all the light would have been reflected back into the diffused regions 76.
Although the array 82 provides for improved output of the light generated in the light-emitting elements, a problem arose With regard to the processing of the array 82. The process for making the projections 78 has instabilities, such as inadequate surface etching and the like. This produces reflection loss from flat portions, decreasing output efficiency and causing variation in emission output.